REVERBERATION; ECHOGRAMS OF INTERIOR SPACES

The purpose of this experiment is to show how multiple reflecting surfaces increase the number of sound paths greatly and how the transition from "exterior" to "interior" acoustics occurs.  To make changes to walls easier, the acoustics are demonstrated in a scale model. Moving full sized concrete walls was too backbreaking.

Place the source and receiver above a single sheet as shown here and fire the spark.

Note the single direct and reflected pulse shown in Fig.18.

There are now four paths from source to receiver as seen in the data in Fig.20.  If the four pulses are not clearly separated, the microphone may be moved around by trial and error until they become distinct.  When this has been achieved, identify the four paths that are taken.  Using a small absorptive pad mounted on the end of a stick block an individual path by moving it around over the wall surfaces until a sound pulse disappears.

Next, add a sidewall as shown in Fig.21 and repeat the experiment. 

There should now be eight paths as shown in Fig.22.  The pulses may not be grouped exactly as shown in the figures; the order in which they appear will depend on the positioning of source and receiver.  Use the same technique as described previously to identify the sound paths.

Adding a second sidewall and removing the end wall, as shown in Fig.23, will cause an infinite row of acoustic images to be formed, shown in Fig.24. 

The row of acoustic images is the similar to that seen when looking into parallel mirrors.  Note that all the acoustical images generally appear in pairs.   They are only equally spaced when the sound source is midway between the walls.

Finally, adding a top to the space and end walls will lead to an extremely dense train of pulses, which becomes a typical reverberant decay when viewed with a slower sweep rate as shown in Fig.25.

It will be noted that the decay rate for the box without a ceiling is somewhat faster than that for the six-sided box (Fig.25).  The cause of this is the one open face of the box acts a “perfect” sound absorber.